Displacement hull ships are very efficient with high Lift/Drag (L/D) ratios up to a point where a very large wave drag component takes president. This is clearly shown upon examination of FIG. 1 of this application that shows the two predominant hydrodynamic resistance or drag components for typical displacement hull ships. These two drag components are friction drag and wave drag. It can be noted from FIG. 1 that friction drag is the predominant drag force up to about 18 knots for a ship with a waterline length of 400 feet (122 meters) and up to about 27 knots for a ship with a waterline length of 800 feet (244 meters). As speed is increased beyond those values, powering requirements become excessive for all practical purposes.
Various attempts have been made to reduce both the friction drag and wave drag of ships. However, though some small improvements have been made, speeds still remain embarrassingly slow for displacement hulls. One of the methods employed to reduce wave drag has been use of a bulbous bow that normally extends forward of the ship's main bow. The bulbous bow operates like a sphere submerged in a moving fluid where the oncoming water tends to adhere to forward and upper and side portions of the sphere and curves inward aft of the largest diameter of the sphere before breaking away in eddies. This inward curving of the fluid on the bulbous bow creates a significant hydrodynamic force on the energy of the bow wave of the ship thereby reducing the amplitude and hence the drag of the bow wave. Bulbous bows are most effective at higher speeds where the wave drag component predominates. Reductions of overall ship resistance values of 5 to 15 percent are noted for ships with well designed bulbous bows. It is of interest that bulbous bows can actually increase drag at low speeds since they increase wetted area friction.
Additionally, wetted area reducing air layers have been applied to the underside of ships and have been shown to reduce resistance by 10–15 percent or more in lower speed operation where friction drag predominates. However, the air layers, while still effective in reducing frictional resistance, are not noted to provide as high a percentage of efficiency improvement at higher speeds where the wave resistance predominates. These air layered ships are normally known as Air Lubricated Ships or simply ALS.
In summary, two successful methods of reducing hydrodynamic resistance of ships are in the prior art. The simple bulbous bow has met with the widest acceptance and is a common feature of larger ships particularly those running at higher speeds. The slightly more complicated ALS requires a blower. The ALS has met with more limited acceptance but does show promise especially for displacement hulls operating at lower speeds.
The instant invention combines bow oriented water propulsor(s) with variations of a secondary bow disposed proximal a lower portion of the ship's main bow. By having a water inlet for the bow oriented water propulsor(s) disposed properly in relation to the secondary bow it is possible to provide an enhanced hydrodynamic force that subtracts from the energy in the ship's bow wave. The effect is to reduce the amplitude and hence the resistance of the ship's bow wave. A related feature is to have a pressurized air or gas layer in a recess in the underside of the hull. This gas layer not only reduces wetted area friction of the ship but also allows the water discharge from the bow oriented water propulsor(s) to be discharged into gas rather than water. The effect of discharging the bow oriented water propulsor(s) into gas rather than water is an increase in the efficiency of the bow oriented water propulsor(s). A further advantage is that a steering and/or reversing system(s) may be applied to the bow oriented water propulsor(s). The steering and/or reversing system(s) would be internal to the pressurized gas recess when moving forward so they do not add to resistance. The advantage of having steering and/or reversing capabilities in the bow makes for a much more maneuverable ship at all speeds.
The instant invention also offers means to reduce stern wave resistance as well as separation and eddy resistance by providing stern oriented water propulsor(s) proximal to and forward of the stern of the ship. This is accomplished by having the stern oriented water inlet(s) properly located. Additionally, the water inlet(s) of these stern oriented water propulsor(s) are conceived so that they may intake the ship's boundary layer water which enhances the efficiency of those stern oriented water propulsor(s). The stern oriented water propulsor(s) would normally have steering and reversing capabilities.
A discussion of the instant invention and the advantages it offers is presented in detail in the following sections.